697 research outputs found
Neutron resonances in planar waveguides
Results of experimental investigations of a neutron resonances width in
planar waveguides using the time-of-flight reflectometer REMUR of the IBR-2
pulsed reactor are reported and comparison with theoretical calculations is
presented. The intensity of the neutron microbeam emitted from the waveguide
edge was registered as a function of the neutron wavelength and the incident
beam angular divergence. The possible applications of this method for the
investigations of layered nanostructures are discussed
Orbital ice: an exact Coulomb phase on the diamond lattice
We demonstrate the existence of orbital Coulomb phase as the exact ground
state of p-orbital exchange Hamiltonian on the diamond lattice. The Coulomb
phase is an emergent state characterized by algebraic dipolar correlations and
a gauge structure resulting from local constraints (ice rules) of the
underlying lattice models. For most ice models on the pyrochlore lattice, these
local constraints are a direct consequence of minimizing the energy of each
individual tetrahedron. On the contrary, the orbital ice rules are emergent
phenomena resulting from the quantum orbital dynamics. We show that the orbital
ice model exhibits an emergent geometrical frustration by mapping the
degenerate quantum orbital ground states to the spin-ice states obeying the
2-in-2-out constraints on the pyrochlore lattice. We also discuss possible
realization of the orbital ice model in optical lattices with p-band fermionic
cold atoms.Comment: 6 pages, 5 figure
Numerical calculation of the combinatorial entropy of partially ordered ice
Using a one-parameter case as an example, we demonstrate that multicanonical
simulations allow for accurate estimates of the residual combinatorial entropy
of partially ordered ice. For the considered case corrections to an
(approximate) analytical formula are found to be small, never exceeding 0.5%.
The method allows one as well to calculate combinatorial entropies for many
other systems.Comment: Extended version: 7 pages, 10 figures (v1 is letter-type version
Magnetic charge and ordering in kagome spin ice
We present a numerical study of magnetic ordering in spin ice on kagome, a
two-dimensional lattice of corner-sharing triangles. The magnet has six ground
states and the ordering occurs in two stages, as one might expect for a
six-state clock model. In spin ice with short-range interactions up to second
neighbors, there is an intermediate critical phase separated from the
paramagnetic and ordered phases by Kosterlitz-Thouless transitions. In dipolar
spin ice, the intermediate phase has long-range order of staggered magnetic
charges. The high and low-temperature phase transitions are of the Ising and
3-state Potts universality classes, respectively. Freeze-out of defects in the
charge order produces a very large spin correlation length in the intermediate
phase. As a result of that, the lower-temperature transition appears to be of
the Kosterlitz-Thouless type.Comment: 20 pages, 12 figures, accepted version with minor change
Entropy-based measure of structural order in water
We analyze the nature of the structural order established in liquid TIP4P
water in the framework provided by the multi-particle correlation expansion of
the statistical entropy. Different regimes are mapped onto the phase diagram of
the model upon resolving the pair entropy into its translational and
orientational components. These parameters are used to quantify the relative
amounts of positional and angular order in a given thermodynamic state, thus
allowing a structurally unbiased definition of low-density and high-density
water. As a result, the structurally anomalous region within which both types
of order are simultaneously disrupted by an increase of pressure at constant
temperature is clearly identified through extensive molecular-dynamics
simulations.Comment: 5 pages, 2 figures, to appear in Phys. Rev. E (Rapid Communication
Benchmarking acid and base dopants with respect to enabling the ice V to XIII and ice VI to XV hydrogen-ordering phase transitions
Doping the hydrogen-disordered phases of ice V, VI and XII with hydrochloric
acid (HCl) has led to the discovery of their hydrogen-ordered counterparts ices
XIII, XV and XIV. Yet, the mechanistic details of the hydrogen-ordering phase
transitions are still not fully understood. This includes in particular the
role of the acid dopant and the defect dynamics that it creates within the
ices. Here we investigate the effects of several acid and base dopants on the
hydrogen ordering of ices V and VI with calorimetry and X-ray diffraction. HCl
is found to be most effective for both phases which is attributed to a
favourable combination of high solubility and strong acid properties which
create mobile H3O+ defects that enable the hydrogen-ordering processes.
Hydrofluoric acid (HF) is the second most effective dopant highlighting that
the acid strengths of HCl and HF are much more similar in ice than they are in
liquid water. Surprisingly, hydrobromic acid doping facilitates hydrogen
ordering in ice VI whereas only a very small effect is observed for ice V.
Conversely, lithium hydroxide (LiOH) doping achieves a performance comparable
to HF-doping in ice V but it is ineffective in the case of ice VI. Sodium
hydroxide, potassium hydroxide (as previously shown) and perchloric acid doping
are ineffective for both phases. These findings highlight the need for future
computational studies but also raise the question why LiOH-doping achieves
hydrogen-ordering of ice V whereas potassium hydroxide doping is most effective
for the 'ordinary' ice Ih.Comment: 18 pages, 7 figures, 1 tabl
The thickness of a liquid layer on the free surface of ice as obtained from computer simulation
Molecular dynamic simulations were performed for ice Ih with a free surface
by using four water models, SPC/E, TIP4P, TIP4P/Ice and TIP4P/2005. The
behavior of the basal plane, the primary prismatic plane and of the secondary
prismatic plane when exposed to vacuum was analyzed. We observe the formation
of a thin liquid layer at the ice surface at temperatures below the melting
point for all models and the three planes considered. For a given plane it was
found that the thickness of a liquid layer was similar for different water
models, when the comparison is made at the same undercooling with respect to
the melting point of the model. The liquid layer thickness is found to increase
with temperature. For a fixed temperature it was found that the thickness of
the liquid layer decreases in the following order: the basal plane, the primary
prismatic plane, and the secondary prismatic plane. For the TIP4P/Ice model, a
model reproducing the experimental value of the melting temperature of ice, the
first clear indication of the formation of a liquid layer appears at about -100
Celsius for the basal plane, at about -80 Celsius for the primary prismatic
plane and at about -70 Celsius for the secondary prismatic plane.Comment: 41 pages and 13 figure
Energy benchmarks for water clusters and ice structures from an embedded many-body expansion
We show how an embedded many-body expansion (EMBE) can be used to calculate
accurate \emph{ab initio} energies of water clusters and ice structures using
wavefunction-based methods. We use the EMBE described recently by Bygrave
\emph{et al.} (J. Chem. Phys. \textbf{137}, 164102 (2012)), in which the terms
in the expansion are obtained from calculations on monomers, dimers, etc. acted
on by an approximate representation of the embedding field due to all other
molecules in the system, this field being a sum of Coulomb and
exchange-repulsion fields. Our strategy is to separate the total energy of the
system into Hartree-Fock and correlation parts, using the EMBE only for the
correlation energy, with the Hartree-Fock energy calculated using standard
molecular quantum chemistry for clusters and plane-wave methods for crystals.
Our tests on a range of different water clusters up to the 16-mer show that for
the second-order M\o{}ller-Plesset (MP2) method the EMBE truncated at 2-body
level reproduces to better than 0.1 m/monomer the correlation energy
from standard methods. The use of EMBE for computing coupled-cluster energies
of clusters is also discussed. For the ice structures Ih, II and VIII, we find
that MP2 energies near the complete basis-set limit reproduce very well the
experimental values of the absolute and relative binding energies, but that the
use of coupled-cluster methods for many-body correlation (non-additive
dispersion) is essential for a full description. Possible future applications
of the EMBE approach are suggested
Free energy landscapes for homogeneous nucleation of ice for a monatomic water model
We simulate the homogeneous nucleation of ice from supercooled liquid water
at 220 K in the isobaric-isothermal ensemble using the MW monatomic water
potential. Monte Carlo simulations using umbrella sampling are performed in
order to determine the nucleation free energy barrier. We find the Gibbs energy
profile to be relatively consistent with that predicted by classical nucleation
theory; the free energy barrier to nucleation was determined to be ~18 kT and
the critical nucleus comprised ~85 ice particles. Growth from the supercooled
liquid gives clusters that are predominantly cubic, whilst starting with a
pre-formed subcritical nucleus of cubic or hexagonal ice results in the growth
of predominantly that phase of ice only.Comment: 11 pages, 6 figures; updated with nucleation rates and additional
comparisons with some newly published paper
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